Semiconductor diode lasers are formed of multiple layers of semiconductor materials. The typical semiconductor laser includes an n-type layer, a p-type layer, optical confinement layers and an undoped active structure between them such that when the diode is forward biased electrons and holes recombine in the active structure with the resulting emission of light. The layers surrounding the active structure typically have a lower index of refraction than the active structure and form a dielectric waveguide that confines the emitted light transversely to the active structure. Semiconductor lasers may be constructed to be either edge emitting or surface emitting.
To confine the emitted light laterally, positive-index guided or negative-index guided (antiguided) structures may be employed in a laser or amplifier diode array. In a positive-index guided structure the refractive index is highest in regions where the laser light has high field intensity and is low in regions of low field intensity, effectively trapping light within the high-index regions, i.e., the laser array elements. In a negative-index guided or antiguided structure, the refractive index is lowest and the optical gain is highest in regions where the laser light has maximum field intensity, i.e., the laser array elements, and the refractive index is highest and there is little or no optical gain in regions that contain relatively low field intensity. Consequently, some of the generated light will pass into the higher refractive index interelement regions and thus will not be confined to the lasing element regions, but this lost light is compensated for by the excess gain in the array element regions.
An array of laser emitters can typically oscillate in several possible modes. In a fundamental or in-phase array mode, all emitters oscillate in phase with each other, and a far field pattern is produced in which most of the energy is concentrated in a single lobe which is ideally diffraction limited. In general, there are many possible array modes for a multiple element array, and many laser arrays operate in two or three array modes simultaneously and produce beams that are typically two or three times wider than the diffraction limit. The problems associated with the operation of laser arrays at high power with high beam quality are discussed in U.S. Pat. No. 4,985,897, entitled Semiconductor Laser Array Having High Power and High Beam Quality. That patent describes a laser diode array structure, which may be implemented in an antiguided configuration, operated at or near the in-phase-mode resonance condition for which coupling occurs equally between all elements of the array.
The development of high-power (greater than one watt) coherent diode laser sources has been an area of continued research efforts. Positive index-guided single-element devices have been demonstrated up to about 0.6 watt (W) continuous wave (CW) coherent power, with reliable operation demonstrated to about 0.2 W, primarily being limited by the relatively small waveguide width of .apprxeq.3 .mu.m. Single-element antiresonant reflecting optical waveguide (ARROW) lasers have also demonstrated single-mode optical power up to .apprxeq.0.5 W, with the added benefit of a drive-independent beam pattern, due to strong lateral optical-mode confinement in devices of 4-6 .mu.m aperture width. See L. J. Mawst, D. Botez, C. Zmudzinski, and C. Tu, "Design optimization of ARROW-type diode lasers,"IEEE Photon. Technol. Lett., Vol. 4, pp. 1204-1206, November 1992. In fact, single-mode ARROW devices with aperture width of up to 10 .mu.m are possible, which should allow for reliable powers of .apprxeq.0.5 W CW from devices with nonabsorbing mirrors.
Research on phase-locked diode laser arrays in an attempt to increase the aperture width and operating power met with little success in controlling the complicated mode structure until the development of resonant-optical-waveguide (ROW) antiguided arrays. Such arrays are described in D. Botez, L. J. Mawst, G. L. Peterson, and T. J. Roth, "Phase-locked arrays of antiguides: Modal content and discrimination,") IEEE J. Quantum Electron., Vol. 26, pp. 482-495, March 1990. Antiguided arrays have demonstrated near-diffraction-limited CW operation at 1 W from a 120 .mu.m aperture, with up to 0.6 W in the central lobe of the far-field emission pattern, and reliable operation over 3500 hours has been achieved at 0.5 W CW output, thus making ROW arrays the only high-power coherent device type to date that has demonstrated long-term reliability. However, since such devices are based on meeting a (lateral) optical resonance condition, the fabrication tolerances on their structural parameters have been experimentally and theoretically determined to be very tight, especially as the number of elements increases. See D. Botez, A. Napartovich, and C. Zmudzinski, "Phase-locked arrays of antiguides: Analytical theory II,") IEEE J. Quantum Electron., Vol. 31, pp. 244-253, February 1995.
A semiconductor laser having antiguide elements and interelement structures with high loss coefficient is described in U.S. Pat. No. 5,606,570 to Botez, et al., entitled High Power Antiguided Semiconductor Laser with Interelement Loss. Such a structure can be used to provide excellent discrimination between the resonant in-phase mode and the unwanted nonresonant modes, and allows relatively large fabrication tolerances.
Currently, in symmetric transverse waveguide lasers, the power can be significantly increased by increasing the waveguide for the same quantum-well(s) size. In turn, the transverse optical confinement factor .GAMMA. (the percent of light energy in the active region) decreases significantly, which, in turn, provides a large equivalent transverse spot size, d/.GAMMA., where d is the quantum-well(s) total thickness. The lowering of .GAMMA. does not significantly affect the threshold-current density as long as the internal loss coefficient is small (1-2 cm.sup.31 1), and the cavity length is increased roughly in the same proportion that .GAMMA. was decreased. Using such structures, very high spatially incoherent powers (e.g., 8-10 W CW) have been achieved from broad-stripe (100 .mu.m) devices. However, since transversely the optical mode hardly penetrates into the cladding layers, it is practically impossible to obtain effective lateral mode confinement for 2-D spatial-mode coherence.